Vehicle surveillance device and vehicle surveillance method

A vehicle surveillance device for an in-vehicle network system that includes one or more electronic control units includes: a frame transmitter and receiver that receives a frame flowing over the in-vehicle network system; and a score calculator that detects a suspicious behavior different from a normal driving behavior based on the frame received by the frame transmitter and receiver and vehicle data including information on one or more frames received by the frame transmitter and receiver prior to receiving the frame, and calculates, based on a detection result, a score indicating a likelihood that reverse engineering has been performed on a vehicle provided with the in-vehicle network system.